1. Field of the Invention
The present invention relates to wear resistant member for electronic equipment in which malfunction due to static electricity is lessened, and a bearing and spindle motor therewith.
2. Related Art
In recent years, development of magnetic recorders such as hard disk drives (HDDS) and floppy disk drives (FDDs), optical disk drives such as CD-ROMs and DVDs, and various kinds of game machines is remarkable. In the electronic equipment, usually, a rotation actuator such as a spindle motor or the like makes a rotating shaft rotate at a high speed, thereby making various kinds of disks attached to the rotating shaft function.
So far, in bearing member supporting such rotating shaft, in particular in bearing balls, metal such as bearing steels or the like is mainly used. However, metal such as the bearing steels is insufficient in wear resistance. Accordingly, in a field where a high-speed rotation of 4,000 rpm or more is required like in the electronic equipment for instance, an increase of dispersion of life results in incapability of providing reliable rotation drive.
To overcome such malfunction, recently ceramic material such as a silicon nitride sintered body is in use for the bearing ball (cf., for instance, Japanese Patent Laid-open Publication No. JP-A 2000-314426). Among the ceramic materials, the silicon nitride sintered body is excellent in sliding properties and has excellent wear resistance. Accordingly, even when rotating at high speeds, rotation drive with mechanical reliability may be provided.
However, the silicon nitride bearing ball is electrically an insulator. Accordingly, there is a problem that when rotating at high speeds, generated static electricity cannot be skillfully liberated to the bearing member other than the rotating shaft or a bearing ball receiver made of metal such as bearing steels. Thus, when the static electricity cannot be skillfully dispersed and is built up unnecessarily on the bearing or peripherals, in a recorder that employs magnetic signals like the HDD for instance, a recording medium is adversely affected. As a result, there are worries that memory in the HDD may be lost, and furthermore the electronic equipment such as the HDD may be destroyed.
In addition, portable personal computers, electronic notebooks and various kinds of mobile products are yearly miniaturized, and to the HDDs or the like used therein demand for higher capacity and more miniaturization is yearly stronger. To cope with such demand, in the HDD for instance, further higher speed rotation is under study, and in the future it is expected to realize a high-speed rotation of 10,000 rpm or more. It is the rotating shaft and the bearing comprising the bearing balls and the bearing ball receiver that supports such high-speed rotation, excessive pressure thereof being substantially concentrated on the bearing balls.
In addition, when rotating at high speeds, on the basis of sliding of the bearing balls, an amount of generated heat (frictional heat) increases. The bearing ball made of the existing silicon nitride sintered body is such low in thermal conductivity as approximately 20 W/mxc2x7K. Accordingly, the frictional heat cannot be effectively dissipated. AS the rotation speed of the bearing becomes higher, lower heat dissipation capability causes more problems. From these circumstances also, the existing silicon nitride bearing ball is said cannot sufficiently cope with the high speed rotation of a long time.
On the other hand, a conductive silicon nitride sintered body of which electrical resistivity is approximately 10xe2x88x925 xcexa9xc2x7m is known (cf. Japanese Patent Publication No. HEI 2-43699 JP-B). Such conductive silicon nitride sintered bodies are used in material for manufacturing a blade or nozzle of a turbine engine by means of electric discharge technique. Therein, a lot of conductivity enhancer such as metal carbides or metal nitrides is added to realize conductive property (lower electrical resistance). In such existing conductive silicon nitride sintered body, though electrical resistance itself may be lowered, much added conductivity enhancer agglomerates with ease, agglomerated particles of the conductivity enhancer causing to lower mechanical strength or the sliding properties of the silicon nitride sintered body.
For instance, in the use where compressive and/or tensile stress is repeatedly exerted to cause fatigue like in the case of the bearing balls, a number of agglomerated particles tend to cause cracks therefrom, resulting in largely deteriorating sliding properties. Accordingly, the existing conductive silicon nitride sintered body as disclosed in the above publication is very difficult to apply in the wear resistant member such as the bearing ball. The above publication does not assume to apply the conductive silicon nitride sintered body in the sliding member, but only assume to enhance electrical conductivity to make use of electric discharge machining.
Japanese Patent Publication No. HEI 7-29855 JP-B, Japanese Patent No. 2566580 and Japanese Patent Laid-open Publication No. HEI 6-227870 JP-A also disclose the silicon nitride sintered body including conductive compounds such as metal carbides or metal nitrides. In Japanese Patent Publication No. HEI 7-29855 JP-B, a silicon nitride sintered body is disclosed in which SiC in the range from 0.1 to 11% by mass is added to a concoction of silicon nitride and rare earth compound. In Japanese Patent No. 2566580 publication, a composite sintered body of silicon nitride and silicon carbide is disclosed. Furthermore, in Japanese Patent Laid-open Publication No. HEI 6-227870 JP-A, a composite sintered body is disclosed in which a dispersant such as silicon carbide or titanium nitride is dispersed in a silicon nitride matrix in three-dimensional network. However, the technologies disclosed in the publications cannot necessarily give the composite sintered body in which appropriate electrical conductive properties and excellent sliding properties are present at the same time.
Furthermore, in Japanese Patent Laid-open Publication No. HEI 8-296649 JP-A, ceramic material having volume resistivity of 104 xcexa9xc2x7m or less is disclosed to apply in a dynamic pressure bearing having a spiral groove generating a dynamic pressure effect. However, ceramic materials used there are sintered bodies mainly composed of conductive ceramics such as SiC, TiC, TiN and TiB2, the composite sintered body of Al2O3 and TiC, and the composite sintered body of ZrO2 and NiO. That is, the ceramic materials in which excellent sliding properties and appropriate electrical conductive properties are present at the same time are not disclosed.
Accordingly, an object of the present invention is to provide wear resistant member for electronic equipment that when applying in bearing balls or the like, may realize a high speed rotation with stability, and in addition suppress unnecessary build-up of static electricity. Furthermore, another object is to provide a bearing and a spindle motor that by the use of such wear resistant member for electronic equipment, enable to realize higher performance and higher reliability of the electronic equipment such as the magnetic recorders like the HDDs and the optical disk drives like the DVDs.
The present wear resistant member for electronic equipment is one that comprises a silicon nitride sintered body containing particles of conductivity enhancer and having electrical resistivity in the range from 1 to 105 xcexa9xc2x7m. The silicon nitride sintered body comprises agglomerations of the particles of conductivity enhancer in which distances between the respective particles of conductivity enhancer are less than 1 xcexcm, the agglomerations of the particles of conductivity enhancer being present in the range of 30% or less by area ratio per unit area in the silicon nitride sintered body.
In the present wear resistant member for electronic equipment, particles of conductivity enhancer to be contained in the silicon nitride sintered body are preferable to be at least one kind of compound selected from carbides and nitrides of 4A group elements, 5A group elements, 6A group elements, 7A group elements in the periodic table, silicon and boron. In particular, at least one kind selected from carbides of tantalum (Ta), titanium (Ti), niobium (Nb), tungsten (W), silicon (Si) and boron (B) is preferably used.
The present wear resistant members for electronic equipment are preferably used in the electronic equipment such as magnetic recorders such as the HDDs and FDDs, and optical disk drives such as the CD-ROMs and DVDs. As a specific shape of the present wear resistant member, a rolling element used in a rotation actuator of the above electronic equipment can be cited. The present wear resistant member is suitable for the bearing balls for the electronic equipment.
In the present wear resistant member for electronic equipment, a silicon nitride sintered body that has high strength and high hardness and is excellent in wear resistance is applied, electrical resistivity in the range from 1 to 105 xcexa9xc2x7m being endowed to the silicon nitride sintered body that is originally an electrical insulator. In addition, the agglomerations of the particles of conductivity enhancer are made to exist by 30% or less by area ratio a unit area in the silicon nitride sintered body.
By compounding the above mentioned particles of conductivity enhancer in the silicon nitride sintered body, electrical resistivity in the range from 1 to 105 xcexa9xc2x7m can be endowed to the silicon nitride sintered body. However, simple compounding of the particles of conductivity enhancer may cause dispersion in the electrical resistivity between the silicon nitride sintered bodies. In the present invention, the agglomerations of the particles of conductivity enhancer are made to exist in an appropriate proportion in the silicon nitride sintered body. By the presence of appropriate amount of the agglomerations of the particles of conductivity enhancer, the electrical resistivity between the silicon nitride sintered bodies may be suppressed from dispersing with reproducibility. When there are a number of agglomerations of the particles of conductivity enhancer, the agglomeration itself becomes a starting point of crack to deteriorate the sliding properties or the like. Accordingly, in the present invention, the agglomerations of the particles of conductivity enhancer are made to exist by 30% or less by area ratio.
In the present wear resistant member for electronic equipment, without damaging characteristics such as mechanical strength, hardness, fracture toughness and wear resistance all intrinsic to the silicon nitride sintered body, appropriate conductive properties are equally endowed to the silicon nitride sintered body. Accordingly, by the application of the silicon nitride sintered body having such characteristics in the bearing ball or the like, in addition to realization of stable high speed rotation, static electricity adversely affecting variously on the electronic equipment can be excellently released.
The present bearing comprises a bearing ball made of the above wear resistant member for electronic equipment of the present invention. The present spindle motor comprises the bearing of the present invention. As a specific shape of the present spindle motor, a structure comprising a fixed shaft, a rotor and a stator can be cited. The bearing is attached to the fixed shaft. The rotor is supported rotatably through the bearing to the fixed shaft and has a rotor magnet. The stator has a stator coil disposed facing the rotor magnet with a prescribed gap.